If you check out the picture you will see small gaps in the top of this print. I have set the multiplier to 1.25 after it usually was 1.1 and prints still have gaps. New nozzle and heater block. Checked the filament and it is 1.75mm. Any ideas?

... its the Extrusion Widht: if the bore are e.g. 5 mm you have only 5 lines to fill the surface, makes an ExtrusionWidth of 1 mm or more. This is as hight as the extruder can not feed enough material. An other thing, a 0,40 mm nozzel has only 0,8 mm surface on the bottom; perhaps you have a mismatch between mm and inches.

Thanks for the replies. This printer is 5 years old and this issue happened off an on. It is a Makerbot Replicator 2. I had been printing with a .3mm nozzle and it was working well. I replaced the nozzle with another .3mm and it would clog up.

So I believe it just isn't building up the correct pressure as someone noted. What could cause that?? Maybe the gears don't grip on the filament as well as they used to. The spring is good to go.

The new nozzle has the same dimensions and the same Material as the old one?
Full metal hotend or PFTE tube?
The hotend was heated up when you have mounted the new nozzle?
Any changes of the hotends cooling or object cooling ?

I had the last days filament material what I couldn't print out with a 0.35 nozzle, with a 0.50 nozzle it runs fine. Whatever the reason was.
But the new 0.35 nozzles are steel and the old one was brass. Can make the difference because they have different coefficient of thermal conduction.

But the new 0.35 nozzles are steel and the old one was brass. Can make the difference because they have different coefficient of thermal conduction.

303 Stainless Steel has 14% the thermal conductivity of 360 brass - so if enough plastic is being pushed through the nozzle a stainless nozzle might not be able to maintain temperature at the nozzle/plastic interface and could require lower print speeds. I haven't printed much with stainless nozzles and haven't seen this in action, but it's certainly a possibility.

Are you using a printed replacement spring loaded lever upgrade that replaces the "squash an o-ring" filament clamp device. If not, get one!

If you are, check the front fan runs all the time. If it doesn't the above printed spring lever will soften and bend.

And if you do have a printed lever, check it has not bent, causing the lever to hit the plastic piece at the top through which the filament feeds, loosening the grip on the filament.

I use a PTFE lined nozzle barrel on both by BotR2s. the liner make the filament run smoothly......but you have to ensure the head cools down sufficiently after printing so the fan cuts out. Don't power down before it stops! Otherwise the PTFE tube will be damaged...and stop the head working.

Thanks for all of the replies. I just ordered a new fan that cools the heat sink fins and a new spring for my filament grip. It is the Makerbot injection molded version that came out a couple of years after the Replicator 2 was launched.